1. Technical Field
This invention relates in general to digital communications and, more particularly, to a GMSK/EDGE modulator with transition smoothing.
2. Description of the Related Art
EGPRS (Enhanced General Packet Radio Service) standard allows mixing consecutive GMSK and 8PSK modulations slot-by-slot within a single TDMA time-frame. To switch between modulation types, the modulator is reset. As shown in FIG. 1, if the modulator is reset in GMSK mode, the I-vector is set to its maximum value, while the Q-vector is set to 0, i.e., the modulator is set to zero phase. If the modulator is reset in 8PSK mode, the phase is initialized close to 0 degrees at a slightly smaller amplitude.
The amplitudes of the signal correspond to √{square root over (I2+Q2)}, where I and Q are the In-phase and Quadrature components of the modulation in the IQ-plane shown in FIG. 1. In the modulator trajectories shown below the 8PSK peak amplitude is the same as the GMSK peak (and rms) amplitude. Therefore the 8PSK rms amplitude will be less than that of GMSK in this case. This is also true when burst data bits are all 1's in both modulations, which is the case during power ramping between bursts where guard bits (user defined as all 1's) are modulated.
At the instant where the modulation type changes, the transition may cause a zero-crossing through the origin of the IQ-plane, because the (I,Q) vector at the end of the previous slot is switched instantaneously to the (I,Q) reset vector of the modulation type in the next slot. In practice, zero crossing in the IQ-plane can cause power amplifier nonlinearity or desaturation. It can deteriorate RF/PA (radio frequency power amplifier) performance and should be avoided.
In the prior art, solutions to this problem rely on reducing the power level of the power amplifier circuit to mask or attenuate such transitions without addressing the problem of the transitions themselves.
Therefore, a need has arisen for a modulator that avoids zero crossings in the IQ-plane when switching between modulation types, for example, between GMSK to 8PSK or between 8PSK to GMSK.